How do I change velocity in the Fracture Modifier?
We, the Fracture Modifier Team, are looking into creative ways to do in future features it but we don't have a workflow for that yet.

It also depends on what look you want to achieve exactly. 
For example, the total mass of the fractured objects could be higher (so the shards have higher masses too). 

Also you could play with gravity, friction, or some force fields to decelerate the shards.

Furthermore you could also try to adjust the time scale of the rigidbody simulation.